Conventionally, sensors for measuring a blood component have been used for clinical tests, self-measurement of blood glucose level by diabetics, etc. The configuration of the sensor for measuring a blood component is such that, for example, a cover is disposed on an insulating substrate having a working electrode and a counter electrode on its surface with a spacer intervening between the cover and the insulating substrate. On the working electrode and the counter electrode, a reagent containing an oxidoreductase, a mediator (an electron carrier), and the like is provided, thereby forming an analysis portion. The analysis portion communicates with one end of a channel for leading blood to the analysis portion. The other end of the channel is open toward the outside of the sensor so as to serve as a blood supply port. Blood component analysis (e.g., analysis of blood glucose level) using the sensor configured as above is carried out in the following manner, for example. First, the sensor is set in a dedicated measuring device (a meter). Then, a fingertip or the like is punctured with a lancet to cause bleeding, and the blood supply port of the sensor is brought into contact with the blood that has come out. The blood is drawn into the channel of the sensor by capillary action and flows through the channel to be led to the analysis portion where the blood comes into contact with the reagent. Then, a redox reaction occurs between a component in the blood and the oxidoreductase so that electrons move to the electrodes via the mediator. A current caused to flow at this time is detected, and the measuring device converts the detected current into the amount of the blood component and displays the value obtained by the conversion.
However, the sensor response of an electrochemical blood glucose sensor as described above may be affected by an interfering substance such as an easily oxidizable compound (e.g., ascorbic acid or uric acid) and the amount of blood cells/hematocrit (Hct). Thus, in order to obtain an accurate measured value, it is necessary to quantitate the interfering substance, the blood cells, or both the interfering substance and the blood cells and then correct the amount of the blood component (e.g., the blood glucose level) based on the value(s) obtained by the quantitation. For example, there has been a sensor that corrects the amount of a blood component by measuring the amount of blood cells by the use of two working electrodes and one reference electrode (see Patent Document 1). Other than this, there has been a method in which the amount of blood cells is measured using a mediator (see Patent Document 2). Also, there has been a method in which an interfering substance is quantitated using an interfering substance-detecting electrode (see Patent Document 3). However, the conventional techniques have a problem concerning the accuracy and the reliability of the measured amounts of the blood cells and the interfering substance so that the amount of the blood component cannot be corrected sufficiently.    [Patent Document 1] JP 2003-501627 A    [Patent Document 2] Japanese Patent No. 3369183    [Patent Document 3] Japanese Patent No. 3267933